iStock/Thinkstock(HAVANA) -- The U.S. is warning the American public against travel to Cuba and cutting more than half the staff of its embassy in Havana following mysterious attacks, including some that have caused symptoms suggesting a sonic assault.

The State Department says 21 embassy employees have been “targeted in specific attacks” and that the withdrawal of all nonessential personnel and all family members from U.S. diplomatic offices in Havana aims to reduce the number of American personnel at risk of exposure.

U.S. officials are also warning Americans about traveling to Cuba.

"The department has issued a travel warning advising U.S. citizens to avoid travel to Cuba and informing them of our decision to draw down our diplomatic staff," Secretary of State Rex Tillerson said in a statement.

Tillerson said the U.S. has no reports of private U.S. citizens being affected by the attacks.

"But the attacks are known to have occurred in U.S. diplomatic residences and hotels frequented by U.S. citizens," he said in the statement.

The U.S. will also halt all official delegations to Cuba and limit short-term travel to only that needed for the investigation, crucial embassy operations, or national security purposes.

JetBlue announced Friday it would waive change and cancellation fees for flights to the Camaguey, Havana, Holguin and Santa Clara airports through Nov. 1.

The embassy in Havana will continue to carry out core functions, but routine visa issuances to people seeking to go to the U.S. will be suspended indefinitely.

An investigation into the attacks is ongoing, and officials said they have not ruled out anyone as responsible, including Cuba or the possibility of a third party like Russia. They also have not ruled out any source, cause, or technology used in the attacks.

The State Department has declined to say if there will be any repercussions for Cuba's presence in the U.S., such as through expelling an equal number of diplomats. In May, the U.S. expelled two senior Cuban diplomats over Havana's failure to protect American diplomats in their country.

Despite the drawdown in Havana, the U.S. said diplomatic relations with Cuba will continue. American and Cuban delegations may still meet in the U.S., for example. Cuba is cooperating with the U.S. investigation and permitting law enforcement officers from the FBI and the State Department's diplomatic security to work in the country.

The latest attack was in late August, but officials said they cannot rule out future attacks either. “We don't know the means, the methods, or how these attacks are being carried out, so I could not categorize them as having stopped in August,” one official said.

The State Department for the first time has also detailed some of the symptoms suffered by victims of the attacks, including hearing loss, dizziness, tinnitus, loss of sense of balance, headaches, fatigue, cognitive issues, and trouble sleeping.

The Foreign Service officers' union has also cited cases of mild traumatic brain injury.